There are a number of silanes that are liquid at ambient temperatures (e.g., from about 15° C. to about 30° C.) or that can be formulated into an ink composition that is liquid at ambient temperatures. Liquid silanes, such as cyclopentasilane or cyclohexasilane, have been investigated as candidate “liquid silicon” precursors. However, to date, it has been challenging to make semiconducting thin films of commercial qualities and quantities from “liquid silicon” precursors. One such challenge has related to doping such “liquid silicon” precursors and/or the films formed therefrom.
Methods have been proposed for covalently binding dopant atoms such as phosphorous and boron to silicon atoms in certain liquid (cyclo)silanes. For example, photochemical reactions between (cyclo)silanes and certain phosphines and/or boranes are disclosed in U.S. Pat. No. 4,683,145 and U.S. Patent Publication No. 2003/0229190. Heterocyclic doped silanes are disclosed in U.S. Pat. No. 6,527,847 and U.S. Patent Publication No. 2003/0045632, and a method for synthesizing such doped silanes is disclosed in U.S. Pat. No. 6,527,847. The properties of thin films formed from such compounds are somewhat disappointing, given the relative proportion of dopant atoms in the film-forming mixture. Also, the results are not quite as reproducible as would be generally desired for commercial applications.
The mechanisms behind the disappointing results are not well understood. However, there may be a number of critical steps involved in forming doped semiconducting films from doped liquid silanes, such as forming the covalent bonds between dopant atoms and silicon, preserving these covalent bonds during subsequent synthesis steps and in initial processing steps to form a thin film or thin film structure, and activating the dopant atoms once the thin film or thin film structure is formed.
Thus, there has been a long-felt need in the art for a “liquid silicon” compound and/or composition, particularly a doped “liquid silicon.” Such a composition would primarily comprise silicon atoms (other than solvent, to the extent any solvent is present as a main component), would include a dopant or dopant precursor, would be liquid at ambient temperatures (to facilitate handling, deposition and further processing), and would yield commercial quality doped semiconducting films upon subsequent processing (e.g., annealing or curing). However, to date, methods of making a thin doped semiconducting film or film structure from liquid silanes have not been sufficiently reliable for high-volume commercial use.